A fuel cell system is configured to generate electricity by supplying a fuel and an oxidizing agent (commonly air) to the anode and the cathode of a fuel cell and causing the fuel and the oxidizing agent to undergo a chemical reaction at the anode and the cathode. Hydrogen is commonly used as the fuel for the fuel cell, but the infrastructure for supplying hydrogen has not been publicly available. For this reason, conventional fuel cell system generally is provided with a fuel processor. This fuel processor generates a hydrogen rich gas by reforming a natural gas (hereinafter referred to as a material), for which the infrastructure has been available, using steam obtained by evaporating water. The resultant hydrogen rich gas is supplied to the fuel cell as the fuel (see, for example, Patent Reference 1).    Patent Reference 1: Japanese Unexamined Patent Publication No. 2001-176528